Vinylidene fluoride-based resins have been widely used in fields requiring long-term durability in order to take advantage of their exceptional weather resistance, heat resistance, contamination resistance, chemical resistance, solvent resistance, mechanical properties and secondary workability. In particular, films consisting of vinylidene fluoride resins are often used as solar cell backing sheets to take advantage of their cost when formed into thin films.
However, the demands on long-term reliability for solar cell backing sheet applications have become stricter, requiring use under severe conditions or longer life. In order to achieve such longer life, a method of adding a UV absorbing agent to a specific layer in a multi-layer structured film has been proposed (Patent Document 1). However, even with such a film, the film can be discolored when exposed to a hot and humid environment for a long period of time. This discoloration signifies the creation of double bonds due to a defluorination reaction in the vinylidene fluoride-based resin, which degrades the appearance and reduces the long-term resistance and long-term reliability.
Therefore, as a method of suppressing film discoloration occurring as a result of prolonged exposure to hot and humid environments as mentioned above, film and sheet materials using highly fluorinated resins typified by tetrafluoroethylene resins have been proposed. However, they cannot be considered to be necessarily suitable for use in solar cell backing sheets due to problems in cost, secondary workability and the like. Patent Document 1: JP H1-262133 A